ACTG 398 - A Phase II, Randomized Trial of Amprenavir as Part of Dual Protease Inhibitor Regimens (Placebo Controlled) in Combination with Abacavir, Efavirenz and Adefovir Dipivoxil Versus Amprenavir Alone in HIV Infected Subjects with Prior Exposure to Approved Protease Inhibitors and Loss of Virologic Suppression as Reflected by a Plasma HIV-1 RNA Concentration >1,000 Copies/ml. This protocol tests the hypothesis that subjects who have failed antiretroviral treatment regimens containing one or more protease inhibitors will experience a virologic response to a regimen utilizing combinations of currently approved protease inhibitors and the investigational agents amprenavir (a nucleotide RTI), and efavirenz (a non-nucleoside RTI), and will tolerate these multidrug regimens. This NIH-sponsored, investigator-intiated study is a Phase II, randomized, partially placebo-controlled, 4-arm trial comparing amprenavir (APV) in a single-PI regimen versus APV in combination with saquinavir (SQVsgc), indinavir (IDV) or nelfinavir (NFV) in HIV-infected subjects currently failing regimens containing IDV, RTV, SQV(sgc or hgc) or NFV, as reflected by a plasma HIV-1 RNA concentration of >1,000 copies/ml. This study will be implemented wih 50% of the enrollment reserved for subjects who have experienced failure on a regimen containing a single PI. All subjects will receive amprenavir (APV), abacavir (ABC), efavirenz (EFV), adefovir dipivoxil (ADV), and L-carnitine supplementation at 500 mg/day. The four regimens are: A. APV 1200mg BID plus SQVsgc 1600mg BID plus ABC 300mg BID plus EFV 600mg/d plus ADV 60mg/d B. APV 1200mg BID plus IDV 1200mg BID plus ABC 300mg BID plus EFV 600mg/d plus ADV 60mg/d C. APV 1200mg bid plus NFV 1250mg BID plus ABC 300 mg BID EFV 600md/d plus ADV 60mg/d D. APV 1200mg bid plus PI Placebo (NFV, IDV, or SQV) BID plus ABC 300mg BID plus EFV 600mg/d plus ADV 60mg/d Subjects will be selectively randomized to one of the four treatment arms based upon prior PI/NNRTI exposure. All subjects will have an equal or greater chance of being randomized to either a dual-PI arm or the single-PI arm (Arm D).